1. Field of the Invention
The present invention relates to a rolling sliding member excellent in lubricating property and a rolling apparatus with the rolling sliding member, and in particular to a rolling sliding member and a rolling apparatus enabling to be suitably served under a condition of acting a large contact stress, an oil free condition, or a micro oil lubrication. The rolling sliding member includes general rolling bearings used to a machine tool or the like, a rolling member suitable for automotive engine parts, oil free bearings served in a vacuum, and a linear guide mechanical parts such as linear guide apparatus or ball screw.
2. Description of the Related Art
In machine tools as a machining center or a high speed lathe, rolling bearings such as ball bearings or roller bearings are used as a bearing for supporting a spindle. This kind of rolling bearing includes an inner ring, an outer ring, a plurality of rolling elements and a retainer. The inner ring is mounted on a shaft to be supported such as a spindle, and the outer ring is provided in the outer peripheral face of the inner ring. The plurality of rolling elements are disposed between the inner ring and the outer ring, and when the inner ring rotates relatively together with the spindle, the rolling elements roll while contacting with the outer peripheral face of the inner ring and the inner peripheral face of the outer ring at surfaces thereof. The retainer holds the rolling elements and rotates in a circumferential direction of the inner ring and the outer ring. Accordingly, in such rolling bearings, frictional heat is generated in contacting parts between both inner, outer rings and the rolling elements, and seizure is probably caused, and so this is prevented with a grease lubrication filling the interior of the bearing with a lubricant such as a grease or the like or a minor oil lubrication such as an oil air.
However, the oil air lubrication generates an air curtain in the interior of the bearing at high speed rotation of the bearing. The air curtain obstructs the lubricant entering into the rolling faces of both inner and outer rings, and brings about a problem of insufficient supply of the lubricant. The grease lubrication is useful for a lubrication of the bearing only, but it has a large agitation resistance of the grease and a large heating value of frictional heat under a condition of high speed rotation, bringing about a problem ready for causing seizure.
In addition, when rotating the spindle at high speed, the lubricant scatters owing to a centrifugal force, and a problem arises, easily causing a boundary lubrication that lubrication is insufficient. In this case, the same problem occurs not only at the contacting parts between the raceway surface of the bearing ring and the rolling elements, but also at the contacting parts between an outer diameter face of the retainer and an inner diameter face of the outer ring and between an inner diameter face of the retainer and an outer diameter face of the inner ring.
Further, recently, a machine tool has intensively demanded a high speed rotation of a spindle for heightening processing efficiency, and based on this occasion, a rotation speed of a roller bearing for spindle has been increased. At the same time, the bearing has been asked to be low heat generation for improving processing precision.
In general, when an angular ball bearing rotates at high speed, a large sliding is caused by spin movement or gyro movement at contacting parts between rolling elements (balls) and raceway surfaces. Further, pressure at the contacting faces between the balls and the raceway surfaces increases owing to centrifugal force acting on the inner ring or the balls and decrease of spaces at the interior of the bearing caused by the difference in temperature between the inner ring and outer ring. The sliding at the contacting parts or the increase of pressure at the contacting faces generate a rising temperature or seizure by heat generation and further various kinds of inconveniences such as advance of excessive abrasion.
As a measure for restraining these inconveniences, it has been practiced to lighten increase of the face pressure effected by the centrifugal force of the balls by reducing the ball diameter or employing ceramics of light weight. JP-A-62-24025 discloses a method of making the material of the inner ring having a smaller linear thermal expansion coefficient than that of the material of the outer ring, so as to control changes in spaces at the interior of the bearing.
JP-A-2000-145749 and JP-A-11-270564 disclose methods of making optimum designs of internal elements in the bearing in order to realize such a bearing of super high speed, high rigidity and low heat generation.
If applying such processed roller bearing to the spindle of a machine tool, an operation is available until about 3,500,000 of DmN value (Dm: pitch diameter mm of the rolling elements, and N: rotation speed min−1) showing allowable rotation speed.
However, the rotating speed of the roller bearing trends to increase more and more, and with only the techniques disclosed above, it is difficult to cope with higher speed and the low heat generation.
In particular, the bearing for the machine tool aims at the lower torque and the low heat generation, and is served under circumstances of minor oil lubrication such as a grease-lubrication, an oil air-lubrication, an oil mist-lubrication or a direct jetting minor oil-lubrication. That is, a lubrication may be effected with a minor amount of lubricant under high speed rotation. Therefore, at the high speed rotation when the DmN value exceeds about 2,000,000, an oil film is broken at the contacting part between the balls and the raceway surface, and both are easy to contact with each other, causing a problem that damages as seizure or abrasion easily occur. Especially, comparing with the oil air-lubrication or the oil mist-lubrication, since the grease lubrication is difficult to form an oil film, the allowable rotating speed is lowered.
On the other hand, in a case that rolling sliding members such as bearings or linear guides are served under oil free conditions as vacuum, clean atmosphere, high or low temperatures, a lubricating manner becomes a problem.
Conventionally, in a vacuum or under a clean circumstance, solid lubricants as Au, Ag, Pb of soft metals, MoS2, and PTFE (polytetrafluoroethylene) have been used for lubrication. Further, under the oil free condition, hard films as CrN or TiN have been often formed on raceway surfaces of inner and outer rings or surfaces of rolling elements for heightening abrasion resistance and seizure resistance.
However, the solid lubricant has a problem that abrasion of the solid lubricant itself is large because the solid lubricant is self-sacrifice. When using the conventional hard film, a life has been very often shortened because a problem is present in a lubricating property.
For solving these problems, such a technique is useful of forming on a metal, a film of a diamond like carbon (hereinafter referred to as “DLC”) having low friction and excellent abrasion resistance and seizure resistance.
The DLC has a surface of hardness equal to diamond, its sliding resistance is small due to frictional coefficient being less than 0.2 similarly to molybdenumdisulfide or fluorine resin. Therefore, the DLC has been employed as a new lubricating material.
For example, in a magnetic disc apparatus, if forming the DLC film of several ten angstroms on surfaces of a magnetic element or magnetic disc, the lubricating property is heightened between the magnetic element and the magnetic disc for protecting the surface of the magnetic disc.
On the other hand, because of its peculiar surface property, the DLC has been noticed as a new lubricating material for rolling sliding members, and recently utilized for imparting the lubricating property to bearings.
For example, JP-W-11-14512 discloses a rolling bearing formed with the DLC film containing a metal in raceway surfaces of the bearing rings or surfaces of the rolling elements. In this rolling bearing, a contact stress is moderated by the DLC film.
Besides, known is the rolling apparatus of the rolling bearing formed with the DLC film on raceway surfaces of the bearing rings or surfaces of the rolling elements by a CVD, a plasma CVD, an ion beam forming process, or an ionization vapor deposition (for example, in JP-A-9-144764, JP-A-2000-136828, JP-A-2000-205277, JP-A-2000-205279 and JP-A-2000-205280).
However, in the rolling apparatus such as the above mentioned conventional rolling bearing, a contact face is a point contact or a line contact having a small area, and since large contact stress acts on the raceway surfaces of raceway rings or the surfaces of rolling elements, the raceway surface of the race way ring or the DLC film is probably broken by repeated stress or shearing resistance.
As seasons of generating such damages, the following three points are assumed.
A first point is that the DLC layer has a property very difficult to deform even if the stress acts on. The DLC is hard and has high elasticity, and if it is covered with a metallic material of small elastic modulus such as a stainless steel or a bearing steel, the DLC cannot follow deformation of a base material owing to difference in both elastic modulus, and the DLC film might be broken.
A second point is that although the DLC is expected to have lubricating property, in comparison with the grease or the lubricating oil, this property is inferior and the DLC is weak in the shearing resistance.
A third point is a problem of embrittlement of the metallic intermediate layer interposed for heightening adhesiveness between copper and the DLC film. That is, because the metal composing the metallic intermediate layer and carbon composing the DLC film are combined to form metallic carbide having brittleness, the metallic intermediate layer is embrittled and the DLC film is easily broken. In case the metallic intermediate layer is composed of one kind of metal, the metallic carbide has large brittleness, and so it is ready for becoming a breaking factor.
In addition, conventionally, for lubrication under circumstances serving bearings disliking steam of a lubricant, or lubrication of rolling members and sliding members serving in a cooling medium, solid lubricants such as fluorine based grease or fluorine resin compound have been employed.
For example, as lubricating technologies for information machinery, known are rolling members dry-adhered on metal surfaces by heating fluoropolyether or fluoropolyalkyl polymer made of fluoropolymer having functional group (for instance, JP-A-10-326468 and JP-A-10-326469).
In the methods described in these publications, the rolling member is immersed in perfluoropolyether (PFPE) or a diluted liquid of a derivative thereof, adhered on the metal surface with PFPE or the derivative thereof, and dry-adhered by heating at 200° C. or lower.
Further, there is also a known method of adhering fluoro derivative having isocyanate group as a functional group onto the surface of the rolling member or the sliding member and heating to cause hardening reaction.
On the other hand, it is difficult to supply enough lubricant to the rolling member and the sliding member used in a cooling medium as a compressor or engine parts, or to sliding members served within engines, to become a boundary lubrication and very often cause seizure. By the way, the lubrication with such a minor amount of the lubricant is small in an agitation resistance of the lubricant, and is a very useful lubricating technique for reducing generation of heat in, e.g., the spindle bearing of the machine tool, but this is difficult to control the amount of the lubricant, and especially in bearings working at high speed, an air curtain (obstructing the lubricant entering into rolling faces) occurs owing to rotation, and becomes the boundary lubrication and very often causes the seizures.
When the supply of the enough lubricant is difficult, a further known method is that endurance of the bearing is lengthened by forming a film made of a material combined with ceramic, metal and alloy on the surfaces of the rolling member and the sliding member. For example, this method is disclosed in JP-A-9-133138. In addition, this publication discloses a technology that a diamond like carbon (DLC) layer having excellent adhesiveness with base materials of the rolling member and the sliding member is formed on the surfaces thereof for imparting a lubricating property to the surfaces of the rolling member and sliding member.
In the rolling member and the sliding member of the bearing, pressure in surface of contacting parts is high, and sliding is caused in the contacting parts. Generally, as an oil film is formed by the lubricant and becomes a liquid lubricant, the contact of metal-to-metal can be avoided. However, the liquid lubricant has a problem that rotation torque or sliding resistance are large by the agitation resistance of the lubricant. Accordingly, a similar problem occurs also in the method of using the solid lubricant as mentioned above.
As the fluorine based grease has a low wettability with metal and a poor oil film-forming ability, it easily becomes the boundary lubrication, and the rolling member and the sliding member using the fluorine based grease have problems about the endurance. Particularly, the usage of the solid lubricant such as the fluorine based compound is useful for improving conformability at an initial period, but not adequate to lubrication of a long time, and its application is limited to the sliding member of very light weighted load.
The technologies disclosed in JP-A-10-326468 and JP-A-10-326469 are that since the liquid lubricating film is difficult to form under a condition where the contacting surface pressure is high as a rolling contact, it easily becomes the boundary lubrication and easily causes the seizure and abrasion. In particular, as the metallic surface is covered with chemically stable metallic oxide, a problem is that the functional group of fluoropolymer is less to combine with metal, and adhesive force of fluoropolymer to the metallic surface is remarkably low.
Explanation will be made to a problem concerned with the above mentioned lubricating method of forming a film made of a material combined with ceramic, metal and alloy on the surfaces of the rolling member and the sliding member, specifically, with reference to the bearings provided with an alloy-steel bearing ring and ceramic-made balls.
If a film of a compound of, e.g., TiN or CrN is formed on the raceway surface of the bearing ring, abrasion resistance in the raceway surface increases. But although having a large affinity with an alloy steel, this compound is harder than the alloy steel composing the bearing ring, and the rolling elements are worn under the boundary lubrication and a preload escapes. Further, since the surface composed of the above mentioned compound has a large sliding resistance, it is difficult to prevent the seizure.
Another example uses hard carbon instead of the above mentioned compound. The hard carbon has hardness equivalent to or higher than the compound as TiN, and is characterized in that coefficient of dynamic friction in the surface is small. However, as the adhesiveness with the metallic surface is weak, the hard carbon layer is ordinarily formed on an intermediate layer made of W, Ti, Si, or Cr for increasing the adhesiveness with the metal as the base material.